This invention relates to a process for manufacturing a piston ring for an internal combustion engine, particularly a piston ring having a wear-resistant layer on its outer peripheral surface.
A piston ring is required to be wear resistant particularly in its outer peripheral surface defining a sliding surface contacting a cylinder liner. It is, therefore, usual to provide a wear-resistant layer 12 on a base piston ring material 11, as shown in FIG. 1. This layer is usually formed by surface coating, which is realized by chromium plating or spray coating of molybdenum, a ferroalloy, ceramics or the like, or heat treatment for hardening, such as soft or ionic bitriding, surface remelting or surface layer melting and alloying. The layer formed by surface coating is, however, not satisfactorily bonded to the base material, but easily peels or breaks. The heat treatment for hardening is low in productivity, since a long time is required, or since some methods, such as surface melting, render the surface so rough as to require posttreatment, though the heat treatment itself may be carried out in a short time.
There are piston rings in which a different material is embedded, for example, a ferrox insert, or a soft metal such as copper or lead. These piston rings are manufactured by filling a groove in a base piston ring material with powder of any such different material, or a paste obtained by kneading it with a binder, and baking the whole. Alternatively, a sheet or ribbon of wire rod material may be press fitted or upset in a piston ring groove.
The material which can be embedded in these piston rings is, however, limited to a soft material which easily undergoes plastic deformation, or a material which can be baked at a low temperature. These materials are primarily employed to improve the draping and lubricating properties of piston rings, and not expected to be resistant to wear.
Accordingly, it is impossible to embed a wire rod of hard material, such as martensitic high-chromium stainless steel, since its difficulty in plastic deformation disables press fitting or upsetting in a piston ring groove, and moreover, since no baking or other method is possible for fixing it to the piston ring.